1. Field of the Invention
This invention relates to an improvement in high voltage field effect and bipolar transistors and in particular to a high voltage MOSFET transistor having a gate electrode semi self-aligned to the channel.
2. Description of the Prior Art
Several examples of prior art high voltage field effect transistors are disclosed in the ECS (Electro Chemical Society) Proceedings Volume 87-13: Proceedings of the Symposium on High Voltage and Smart Power Devices, May, 1987, pages 1-67.
The high voltage P channel devices disclosed in the above-cited prior art share several characteristics:
1. All use thin gate oxide, not thick oxide.
2. All use some form of drain extension to obtain high breakdown voltage.
3. All have source metal extended over and past the edge of the gate towards the drain.
4. None use a deep P- region on the source side.
A problem encountered with the prior art is that the prior art transistors generally use a "Drain Extension" region which is not self-aligned to the source, which results in variations in channel length and thus variations in output characteristics. The polysilicon gate is also not self-aligned, thus requiring adequate overlap to be built into the design rules, and thus requiring a larger device.
A second problem is that in the prior art the "Drain Extension" region may require an additional mask step in the fabrication process.
Another prior art device is disclosed by Tadanori Yamaguchi and Seiichi Morimoto in "Process and Device Design of a 1000-Volt MOS IC" in 1981 Proceedings of the IEEE IEDM (International Electron Devices Meeting) pages 255-258. Yamaguchi et al. disclose a MOS transistor which can maintain a breakdown of 1000 volts. Yamaguchi et al. use an implanted extension of the source region and an extension also of the drain region. (see FIG. 1) The channel is self-aligned with the source region and with the drain extension. Field oxide covers both the source extension and drain extension regions, and the source metallization covers the gate region. The gate oxide is 1050 .ANG. thick. Yamaguchi et al.'s drain extension region has two subregions, with differing dopant concentrations.
The extension of source metal over the gate is disclosed in Yamaguchi et al., supra. This metal extension serves as a field plate, allowing the high electric fields to be dropped across the thick oxide between drain and the field plate rather than the thin oxide between drain and gate, thus preventing the field plate (poly gate) induced breakdown due to the thin gate oxide.
The prior art devices exhibit breakdown voltages from 180 to 1000 volts. Micrel Inc. has fabricated and sold its MPD 8020 and MPD 8030 High Voltage integrated circuits. Both of these products are fabricated by a high voltage P channel (HVPCH) process with a standard field oxide as gate oxide. The MPD 8020 and the related process are disclosed in copending U.S. patent application No. 07/236,656, entitled "User Configurable Integrated Power Circuit," filed Aug. 25, 1989, inventors: Zinn, et al., now abandoned.